The present invention relates to a negative pressure booster for brake boosters and, in particular, to a negative pressure booster provided with a diaphragm, where the bead portion of the diaphragm is squeezed between a front shell and a rear shell.
Many types of brake booster utilizing negative pressure have been used on automobiles in the past in order to obtain a higher braking power with a smaller pushing force on brake pedal. A typical example of a brake booster 1 is shown in FIG. 3.
As it is evident from FIG. 3, this brake booster 1 is has a front shell 2 and a rear shell 3. A valve body 4 passes through the rear shell 3, and the valve body 4 is airtightly and slidably supported on the rear shell 3. A power piston 5 is provided between the valve body 4 and each of the shells 2 and 3, and the power piston 5 is provided with a diaphragm 6. The space formed by the shells 2 and 3 is divided into a constant pressure chamber 7 and a variable pressure chamber 8 by the diaphragm 6. The constant pressure chamber 7 communicates with the intake manifold of engine (not shown) through a negative pressure leading pipe 9, so that negative pressure can be introduced at all times.
In the cavity of the valve body 4, a control valve 10 is provided, and this control valve 10 controls the communication or the interruption of the variable pressure chamber 8 and the constant pressure chamber 7 with atmospheric air. The control valve 10 is controlled by an input shaft 11 connected to a brake pedal (not shown). Further, on the forward end of the valve body 4, an output shaft 13 is slidably mounted through a reaction disk 12, and this output shaft 13 passes airtightly and slidably through the front shell 2 and protrudes outside the brake booster and operates the piston of brake master cylinder (not shown).
When not operating, the variable pressure chamber 8 is at least shut off from the atmospheric air. When the brake pedal is depressed, the input shaft 11 moves forward, and the control valve 10 is switched over. Then, the atmospheric air flows into the variable pressure chamber 8 and exerts action on the power piston 5. Power piston 5 is thus operated, and by the action of this power piston 5, output shaft 13 moves forward and operates the piston of the master cylinder (not shown). As the result, the braking action is performed.
In such type of brake booster 1, the two shells 2 and 3 are connected together generally by holding the peripheral portions of front shell 2 and rear shell 3 together. In this case, a bead portion 6a of diaphragm 6 is present between the peripheral portions of two shells 2 and 3, and the bead portion 6a is squeezed by two shells 2 and 3.
As shown in FIG. 4, the bead portion 6a of the diaphragm 6 is furnished with an outer lip 6b, and this outer lip 6b prevents the leaking of atmospheric air into the constant pressure chamber 7.
However, when the front shell 2 and rear shell 3 are assembled in such conventional type brake booster 1 as shown in FIGS. 5(a)-5(c), the front shell 2 is moved rightward as shown by an arrow "a" with the bead portion 6a of diaphragm 6 placed between peripheral portion 2a of front shell 2 and peripheral portion 3a of rear shell 3. Then, the first step 2b formed on the front shell 2 is brought into contact with the outer lip 6b as shown in FIG. 5 (b). When the front shell 2 is moved toward the direction of the arrow "a" under such condition, the bead portion 6a begins to rotate in the direction of the arrow "b". If the front shell 2 is further moved in the same direction a, the bead unit 6a is extensively rotated as shown in FIG. 5 (c). This results in outer lip 6b being pulled toward the marginal flange 3b of the rear shell 3 and being squeezed between the first step 2b of the front shell 2 and the marginal flange 3b of the rear shell 3. As the result, outer lip 6b is often damaged or broken.
As shown in FIG. 5 (c), the rolling portion 6c of the diaphragm 6 is also squeezed between the second step 2c formed by the front shell 2 and the folded portion 3c of the rear shell 3. Thus, the rolling portion 6c is often damaged or broken.
Therefore, care must be taken not to break the diaphragm 6 when front shell 2 and rear shell 3 are assembled. This requires much time for assembling, thereby reducing working efficiency.